1. Field of the Invention
The invention relates to the field of metallurgy and particularly to the field of Al-Zn-Mg-Cu alloy having high strength and high corrosion resistance.
2. Description of the Prior Art
Aluminum alloys are widely used in structures wherein low weight and high strength properties are required as those of airplanes.
Among them the 7000 series Al-Zn-Mg-Cu aluminium alloys represented by 7075 and 7050 aluminium alloys of Japanese Industrial Standard (JIS) are widely utilized. These alloys obtain high strength by fine precipitates resulting from solution heat treatments and aging treatments. Generally speaking in the aging treatment, alloys are heat-treated under isothermal conditions for from several hours to a duration of a time less than 100 hours in the temperature range of from 100.degree. to 200.degree. C. at single or dual temperature level. For example, in the recommended aging condition of JIS-W-1103, the temperature range is from 116.degree. to 127.degree. C. and the aging time is 24 hours for the 7075 alloys, whereas for the 7075 alloys with T 73 treatment, the temperature range is from 102.degree. to 113.degree. C. and the aging time is from 6 to 8 hours for the first step treatment and from 102.degree. to 113.degree. C., and from 6 to 8 hours for the second step treatment. In the aging treatment, the temperatures should be kept constant in the recomended range for the duration of a comparatively long time, which leads to the prescribed material properties of the alloys.
In the 7000 series aluminium alloys, the high strength property is obtained by the formation of the fine precipitates of the aforementioned solution treatment and the aging treatment.
However the dimension, shape, and distribution of the precipitates vary with the aging condition. For example, in the case to 7075T6 alloy, the tensile strength of 58 kgf/mm.sup.2 is obtained, whereas susceptibility of the stress corrosion cracking is enhanced. In the case of the 7075 alloys forging, the threshold stress in the ST direction wherein the stress corrosion cracking does not occur in the material, is 6 kgf/mm.sup.2 for 7075T6, T6, and 31 kgf/mm.sup.2 for 7075T73 condition. The resistance to the stress corrosion cracking of the material is enhanced at the sacrifice of the strength of alloys. Accordingly it is difficult to have both the corrosion resistance and the high strength property of material in the 7000 series aluminium alloys. The cause lies in the state of the precipitates which is determined by the aging treatment. When the aging is carried out under a comparatively low temperature such as 120.degree. C., a very fine precipitate of the size of 5 nanometers is formed, and high strength is obtained. When the aging is carried out under a comparatively high temperature such as 170.degree. C. as in the case of 7075 T 73, the size of the precipitate grows to from 10 to 20 nanometers, and the strength is lowered, but the corrosion resistance such as the susceptibily to stress corrosion cracking is lowered.
As mentioned above, to produce aluminium alloys having both the corrosion resistance and the high strength property, it is necessary to change the state of the precipitates. However, it is difficult to change the state of the precipitate in the prior art.